An ignition system of an internal combustion engine found in the prior art is shown in FIG. 2. The circuit comprises a Darlington transistor 23, a thyristor 24, resistors R1 through R6, a protection diode 27, an ignition plug 25, a zener diode 26, and an ignition coil 2. The ignition coil 2 comprises a primary coil 21, a secondary coil 22, and terminals A and B. As depicted therein, the Darlington transistor 23 and the thyristor 24 are connected in series to the resistor R1 across the primary coil 21 of the ignition coil 2. When a voltage is generated in accordance with the rotation of a flywheel magnet at terminals A and B of the primary side of the ignition coil 2, terminal A is rendered positive with respect to terminal B. A base current of the Darlington transistor 23 thereby flows through the resistor R1 and a collector current also flows in accordance with the amplification factor of the Darlington transistor 23. When the voltage at terminal A increases further, the thyristor 24 turns ON with a voltage determined by the voltage divider formed by resistors R2 and R3. Resistors R2 and R3 can be adjusted so that the voltage at terminal A is at a certain desired value when this occurs. When the thyristor 24 turns ON, current ceases flowing into the base of the Darlington transistor 23, switching the Darlington transistor 23 OFF. Thereby, a voltage as high as several hundred volts is generated at the primary side of the ignition coil 2, enabling the ignition plug 25, provided at the end part of the secondary coil 22, to fire.
The zener diode 26 is provided to protect the Darlington transistor 23 from damage resulting from high voltages. In addition, when terminal B of the ignition coil 2 becomes positive with respect to terminal A, the protection diode 27 and the resistor R4 suppress the voltage to be applied to the Darlington transistor 23 from the terminal B so as to prevent misfiring of the ignition plug 25.
In the prior art circuit of FIG. 2, a small voltage is generated in the primary side of the ignition coil 2 as the internal combustion engine revolves at a low speed, and voltages on the order of two volts are generated during operation at 400 to 500 revolutions per minute (RPM). It has therefore been difficult to obtain the energy necessary for ignition at low speed if the amplification factor of the Darlington transistor 23 is not large enough. However, if the amplification factor of the Darlington transistor 23 is too large, the collector-emitter voltage is lowered, and secondary breakdown is generated, resulting in serious adverse effects on the circuit at higher speeds.
As a means for solving the problems described above to economically obtain the energy required for ignition, even during low speed revolution of an internal combustion engine, an insulated gate semiconductor device, such as an insulated gate bipolar transistor (IGBT) could be considered in place of the Darlington transistor. Consideration could also be given to use of a power metal-oxide semiconductor field effect transistor (MOSFET) having main electrodes on both side surfaces and having the insulated gate structure on one surface. However, in order to drive the IGBT or MOSFET, it would be necessary to bias the device with a voltage that is higher than the gate threshold voltage. This voltage is a function of the transmission characteristic across the gate and emitter (for the IGBT) or across the gate and source (for the MOSFET). In general, a voltage of 5 volts or higher would be required for such biasing.
In such a general biasing method, the biasing voltage generated at low revolution speeds in the prior art circuit is too low to drive the IGBT or MOSFET. Although additional driving circuits can be provided, such additional circuits are uneconomical because of additional circuit components and complexity required.
Accordingly, it is an object of the present invention to provide a method for driving or biasing an insulated gate semiconductor device such as an IGBT or MOSFET without additionally providing a driving circuit, even when only a low voltage is available across the gate and emitter of the IGBT or across the gate and source of the MOSFET.